The field of the present invention is processes for purifying terephthalic acid.
The state of the art of methods for purifying terephthalic acid may be ascertained by reference to U.S. Pat. Nos. 3,660,478; 3,859,344 and 3,887,612 and British Pat. Nos. 982,629; 1,237,786 and 1,454,478, the disclosures of which are incorporated herein.
As is known, a particularly pure terephthalic acid (TPA) is required for the production of fibers, sheets, films, or also injection-molding compositions of polyesters of terephthalic acid, for example of polyethylene terephthalate, in order to obtain the desired film- and fiber-forming properties, high melting points, pronounced crystallinity, and complete absence of color.
Crude TPA as obtained, for example, in accordance with the conventional oxidation process for p-xylene in acetic acid on heavy metal ions and bromine ions with the use of air, is, however, contaminated generally by oxidation intermediates, such as p-toluic acid and terephthalaldehydic acid (TPAA), as well as by catalyst salts. While the catalyst residues and p-toluic acid can be readily removed by leaching with warm acetic acid, the TPAA resists such a treatment. Since TPA is hardly at all soluble in solvents and melts only at above 425.degree. C. under decomposition, the customary purifying methods, such as, for example, recrystallization or distillation, cannot be readily employed. However, the TPAA can be present in the TPA up to an extent of 3-5% and causes the polyester to assume a deeply dark coloring, and furthermore lowers the melting point and crystallinity of the polyester considerably.
For this reason, various purifying processes have been suggested for TPA, and among these also methods employing a secondary oxidation.
In accordance with British Pat. No. 982,629 crude TPA is to be subjected to a secondary oxidation step in acetic acid at 250.degree. C.; in this process, the TPAA contained in the solid TPA particles is dissolved and can be oxidized. However, this method is uneconomical, because at above 200.degree. C. the acetic acid is degraded by oxidation, on the one hand, and the installations are corroded, on the other hand. The process cannot be operated usefully, though, at below 200.degree. C., because the solubility of the TPA is too low; a purified TPA is obtained in this case only if the crude TPA has already a very high degree of purity.
In conformance with the above, British Pat. No. 1,454,478 teaches that a purifying effect is achieved by way of the secondary oxidation of a TPA slurry in acetic acid at 190.degree.-195.degree. C. with air only if the first oxidation stage has left only a small amount of TPAA in the TPA; however, to attain this objective, a long oxidation period is required at a high temperature (for example 175 minutes at 210.degree. C.), and the required time period can only be shortened somewhat by further increasing the temperature or the degree of dilution. The purifying effect, though, is only 66%, even under optimum conditions.
Quite similarly, U.S. Pat. No. 3,859,344 discloses that a crude TPA containing 570 p.p.m. of TPAA can be purified to 370-290 p.p.m. TPAA by subjecting the crude TPA to a secondary oxidation and a residence time in several crystallizers. The purifying effect is, accordingly, only 35-48% and here again there is the prerequisite that the crude acid be already relatively pure.
U.S. Pat. No. 3,887,612 seeks to overcome these disadvantages by recommending a pumping and agitating treatment instead of the secondary oxidation. In this process, the crude TPA is made into a slurry with acetic acid, recirculated for about 3 hours by means of a pump, which reduces the particle size of the TPA by 10-30%, heated to 130.degree.-180.degree. C., and separated. The most advantageous example, however, shows a purification of the TPA of only from 90 p.p.m. to 30 p.p.m. TPAA. The reference conveys the further teaching that a comminution of the TPA particles of more than 30% is to be avoided, since the impurities are concentrated in the close proximity to the surface of the particles; consequently, the pumping and agitating process has the purpose of moderately abrading the surfaces. In contrast thereto, if the particles are reduced by, for example, 60%, then too many particles are produced having a very large surface area in total, which perforce adsorb the TPAA dissolved in the acetic acid. Here again, therefore, the process has limitations in that the concentration of TPAA in the dispersant determines the concentration of the TPAA in the TPA separated from this dispersant. Thus, this process also requires a crude TPA which is already pure.
U.S. Pat. No. 3,660,478 proceeds along still another path by proposing to circulate the slurry of crude TPA in acetic acid through a crystallizing loop at least ten times alternatingly in a temperature interval of 0.5-17.degree. C. and to withdraw a small portion for processing, the heat being removed by evaporative cooling. Per each cycle, 1-10% of the dissolved proportion of TPA is to be recrystallized. The reference sets forth a degree of purity of up to 90%; however, since the starting material is to be crude acid containing up to 5% TPAA and p-toluic acid, the degree of efficiency of the proposed process is entirely unsatisfactory.